Basic principles of thermometers and how to use them

Basic principles of thermometers and how to use them2020

Australian

Part 1

COLDFOODCODE

ColdFoodCode Cat No.Part1.Rev2.07122020

AUSTRALIAN COLD FOOD CODE

The Australian Cold Food Code comprises five parts:

Part

1Basic principles of

thermometers and how to use them

Part

2Guiding principles of fresh produce

transport and storage

Part

3Guiding principles of food cold chain

transport and storage

Part

4Cold chain

temperature monitoring

requirements and specifications

Part

5Thermal

requirements of refrigerated

transport assets

Devised in association with

Research, writing and editing teamMark Mitchell – Chairman Australian Food Cold Chain Council (AFCCC) and Managing Director SuperCool Group of Companies Dr Hanan Hamid – Scientific and Technical Services Manager SuperCool Asia Pacific Pty LtdDr Greg Picker – Executive Director Refrigerants Australia (RA) and Executive Director Australian Food Cold Chain Council (AFCCC)Dr Peter Hofman, Senior Principal Horticulturist, Horticulture and Forestry Science, Department of Agriculture and Fisheries, Queensland GovernmentKen and Bron Newton – Newton Corporate Communicators and Australian Food Cold Chain Council (AFCCC) CommunicationsJulia Collins – former Industry Policy Adviser National Road Transport Association (NATROAD)Peter Lawrence – Technical Director ANZ Thermo King

With acknowledgement to the original Code of practice for the road transportation of fresh produce (1996) produced by the former Australian United Fresh Transport Advisory Council (AUFTAC)Author – Anne Story, Post-harvest Researcher

Copyright © 2020 All rights reservedNo part of this document shall be reproduced in whole or in part without permission. This includes reproduction or copies in any form or by any means including photocopying, printing or electronic media.

DISCLAIMERThis document provides guidelines to the basic principles and technologies of thermometers typically used in the cold food chain. While reasonable efforts have been made to ensure that the contents of this publication are factually correct, the authors do not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication.

TABLE OF CONTENTS

1. Introduction 5

2. Basic concepts 6

2.1 Probe thermometers and infrared sensors 7

2.2 Time-temperature recording devices 7

2.3 Single-use temperature indicators 7

3. Types of thermometer 8

3.1 Thermistor 8

3.2 Thermocouple 8

3.3 Resistive temperature detector 8

3.4 Infrared thermometer 9

3.5 Bimetallic device 9

3.6 Change-of-state sensor 9

3.7 Silicon diode sensor 9

4. Thermometer selection 12

4.1 Non-destructive temperature measurement 12

4.2 Destructive temperature measurement 18

5. Measurement procedures and techniques 19

5.1 Calibration 19

5.2 Where to place thermometer probe in food 21

5.3 Factors that affect measurement accuracy 24

6. Importance of temperature control 25

LIST OF TABLES 1 Types of thermometer 10

2 Emissivity values of objects likely to be found in food packaging 13

3 Criteria for selection of working-standard thermometers 20

4 Probe insertion into whole protein foods 22

5 Fresh produce comparisons depending on measurement method 24

6 Summary of shelf-life guidelines for packaged meat and meat products 26

7 Length of time foods at risk can be held safely at temperatures above 5°C 26

LIST OF FIGURES 1 The most common sensors used in thermometers in the food industry 11

2 Thermometer measuring the temperature 12

3 Distance-to-spot ratio in infrared thermometers 15

4 Comparison between infrared measurement of low and high emissivity surfaces and how to compensate for reflective surfaces with electric tape 16

5 Example: Measuring the temperature of yoghurt in a small pot using an infrared sensor 17

6 Example: Measuring the temperature of sausages in a foam tray using an infrared sensor 17

7 Ice point calibration 20

8 Measuring temperature of fresh chilled products 21

9 Some right and wrong thermometer placements 22

10 Flowchart for temperature measurement process 27

Australian Cold Food Code

4

1 . INTRODUCTION

Temperature measurement and monitoring are critical requirements of the cold chain and measurement accuracy is vital for effective cold chain management and compliance.

The safety and quality of chilled and frozen food during transport from primary production to the distribution centre and retailer depend on maintaining the correct temperature of the food to prevent the growth of pathogens and minimise the growth of spoilage microorganisms. Safe temperatures can vary, depending on the product.

Basic principles of thermometers and how to use them is one of five parts of the Australian Cold Food Code.

This part provides guidelines on the types, selection and calibration of thermometers that are typically used in the chilled and frozen food industry. It includes details of the measurement process as applied to different food types and examples of different measurement scenarios and processes for refrigerated products.

The Australian Cold Food Code builds on the integrity of the cold food chain, on which the quality, safety and shelf life of produce depend. The correct use of thermometers to measure product temperatures at points of dispatch, on delivery, and at all critical control points in the chain, such as loading docks, is the only means of recording and proving the integrity of the cold chain. Maintaining the integrity of the cold food chain also limits exposure to food loss and wastage with its financial and environmental consequences.

Basic principles of thermometers and how to use them

5

2 . BASIC CONCEPTS

The aim of this document is to provide those responsible for food temperature management with an understanding of the most common temperature measuring technologies which will guide the user to select the best thermometer for the purpose.

This document does not contain recommendations for any particular type or brand of thermometer. Because of the enormous range of temperature-sensitive food types, along with the even larger variety of sensors, thermometers and probes available on the market, the onus rests on the user to determine the most appropriate type of thermometer for the purpose.

Also covered are explanations and diagrams illustrating how temperature probes should be used in various food types in order to deliver the most accurate temperature measurements.

All chilled or frozen foods have their own safe temperature specifications, so when selecting a type of thermometer, it is essential to clearly understand the following parameters:

■ type of food being carried

■ type of packaging – bulk on pallets, in boxes, cling-wrapped

■ state of the food – chilled or frozen

■ target temperature – centre or surface

■ measurement accuracy and response time

■ thermometer sensor stability over a period of time

■ ease of use and cost effectiveness.

Thermometers of various types, using a variety of thermal sensors, are used to measure temperatures but the most common types comprise two main elements:

1. a sensor that measures temperature by resistance changes

2. a means to convert those changes into a numerical value.

In general, there are four methods for measuring food temperatures in the cold chain:1. placing thermometers with probes close to, or between, boxes or pallets

2. using hand-held thermometers with probes that are inserted into food

3. using time-temperature recording devices fitted with probes and sensors

4. attaching single-use temperature indicators to food packaging.


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2.1 PROBE THERMOMETERS AND INFRARED SENSORSProbe thermometers or infrared sensors are typically used to measure temperatures at the warehouse or loading dock. The temperatures are accessed using WiFi logger, Bluetooth, LCD display or a data logger wired to the thermometer or sensor.

2.2 TIME-TEMPERATURE RECORDING DEVICESTime-temperature recording devices are typically used to monitor food and ambient temperatures during transit. Methods of data collection and retrieval can vary.

■ Data from some devices can be retrieved manually by connecting the logger to a computer to download the recorded temperature. However, it does not provide real time temperature tracking where the data is directly sent to a separate data logger or web portal wirelessly using SIM card or Bluetooth.

■ Some data loggers record the location of the monitored product, the humidity in the cold space and the truck speed, whether it is moving or stationary, and the number and times of door openings.

■ The data logger can be small, with a built-in temperature sensor. It can be placed inside a package or box or can be connected to an external analogue sensor or probe. The same configuration can be used where the recorder is some distance from the product (depending on the length of the analogue sensor wire) such as warehouse racking or truck driver cabin. The recorded data can be accessed by PC, tablet or smartphone with alarms using SMS or email.

■ The temperature data logger can be a single-use device to monitor the product temperature during transportation. This type of logger can be programmed for logging intervals and alarms at the beginning of the trip only. The data can be downloaded when it is connected to a PC and some have single SIM loggers where the logging interval can be changed during transit. They don’t need to be connected to a PC to download the data. These types are usually compact, lightweight and waterproof so they can be easily placed inside the product package.

2.3 SINGLE-USE TEMPERATURE INDICATORSSingle-use temperature indicators are temperature-sensitive labels that are attached to food packaging. They change colour if the temperature exceeds the maximum safe temperature during transport and storage.

This technology is used to show changes in product temperature ranges. Non-reversible temperature labels are adhered to the product package to indicate if the temperature has exceeded its safe temperature during transport or storage. The colour of the label changes when a certain temperature is reached (±1°C accuracy) but it cannot provide the current temperature, nor the time that the temperature change occurred.

Other types track temperature breaches across the cold chain and record the duration of the breach. For example, if the product alert temperature is 5°C and the product was out of temperature for two hours during the journey, the indicator will show it.


7

3. TYPES OF THERMOMETER

There are seven main technologies that make thermometers functional, and each has different operating principles (Table 1 and Figure 1).

More common technologies Less common technologies

ThermistorThermocouple

Resistive temperature detectorInfrared thermometer

Bimetallic device

Change-of-state sensorSilicon diode sensor

3.1 THERMISTORThe thermistor is an electrical resistor whose resistance is altered by heating and is used for measurement and control. There are two types of thermistor, depending on the resistance changes with the temperature:

■ negative temperature coefficient (NTC) – the resistance decreases when the temperature rises

■ positive temperature coefficient (PTC) – the resistance increases when the temperature rises.

The NTC model is the most common for measuring temperatures in thin and thick foods.

It is very sensitive and precise, with a very tight tolerance and limited temperature range compared to the PTC. However, the PTC model is more stable, has a rapid response and a wider temperature range.

3.2 THERMOCOUPLEThe thermocouple has a very small junction formed from a pair of welded metals with different thermo potentials. The principle is based on voltage generation due to the junction temperature changing. The voltage is increased with temperature rise. There are eight types of thermocouple: J, K, E, T, N, B, S and R, and each type has its own unique characteristics in terms of accuracy, temperature range, durability, vibration resistance, chemical resistance and application compatibility. For example, K type is a general-purpose thermocouple with a wide temperature range and is used across many industries and processes. The J type is used to monitor temperatures of inert materials and in vacuum applications and the N type is suitable for high temperature monitoring.

The K type (nickel–chromium/nickel–alumel) is the most commonly used model. It is accurate, reliable and inexpensive and works in a wide range of temperatures.

3.3 RESISTIVE TEMPERATURE DETECTORThe resistive temperature detector (RTD) measurement process correlates the resistance of its element with the temperature. The RTD has a very accurate measurement and small time constant compared to the thermistor but lower thermal sensitivity (1Ω/°C). The platinum resistive thermometer is the most accurate RTD sensor.


8

3.4 INFRARED THERMOMETERThe infrared thermometer is a thermoelectric device that converts radiant heat energy into electrical potential. The temperature is measured by detecting the infrared radiation/emitting heat from the object in the form of rays and funnelling the light from the rays into a detector. It is used for non-contact temperature measurement.

3.5 BIMETALLIC DEVICEThe thermal expansion property of the bimetallic strip metal converts the temperature to a mechanical displacement, so the expansion and contraction of the strip metal changes with the variation in temperature. There are four common forms of the bimetallic thermometer: spiral, helix, cantilever and flat.

3.6 CHANGE-OF-STATE SENSORChange-of-state temperature sensors measure a change in the state of an object, brought about by a change in the object’s temperature. Commercially available sensors are in the form of labels, pellets, crayons, lacquers or liquid crystals. Their accuracy is low and the response time is slow, sometimes taking minutes to respond.

3.7 SILICON DIODE SENSORThe silicon diode sensor specifically measures temperatures in the very low temperature range (low temperature applications at or below -150°C) with high accuracy and fast response.

Essentially, it is a linear device where the conductivity of the diode increases linearly in the low cryogenic regions. Generally, this sensor can be used for static and dynamic temperature measurements in the -271°C to 176°C temperature range


9

−55°C to 150°C dependent on type

-200°C to 1250°C dependent on type −260°C to 850°C –30°C to 500°C −75°C to 1500°C

ThermistorFigure 1a

ThermocoupleFigure 1b

Resistive temperature detector

Figure 1c

Infrared thermoelectric sensor

Figure 1dBimetallic device

Figure 1e

Constant voltage or current Self-powered Constant voltage or

currentConstant voltage or

current Self-powered

Power requirement

12mm or deeper for varying product

thickness 6mm Immersed for varying

product thickness Non-contact 50mm or to suit thickness of products

Placement

High Low LowModerate to high, depending on the

surfaceLow

Sensitivity0.12–10 seconds 2–5 seconds 2–5 seconds <2 seconds 20 seconds

Response speed

■ Nonlinear output ■ Limited temperature range

■ Time constant* is twice that of the resistive temperature detector

■ Not all models can be calibrated

■ More susceptible to electromagnetic interference

■ Less stable or accurate compared with the resistive temperature detector

■ Poor linear response

■ Susceptible to cold junction compensation

■ Low sensitivity ■ Heat generated by current passing through the resistance can cause measurement error

■ Maximum measurement distance 5cm

■ Not accurate ■ Affected by surrounding conditions such as frost, moisture, dust, fog, smoke, air particles and rapid changes in ambient temperature

■ Only measures surface temperatures

■ Reflective surfaces affect accuracy

■ Can be affected by radio frequency with an electromagnetic field strength of 3 volts per meter or greater

■ Cannot see through glass, liquids or other transparent surfaces – if pointed at a window, it will measure the window pane temperature

■ Requires frequent calibration

■ Fairly slow response ■ Accuracy is ±2% to ±5% of the scale, less accurate at low temperature

■ Takes an average of temperatures

Disadvantages

■ Durable ■ Long lasting ■ Highly sensitive ■ Small size ■ Measures single point temperature

■ Can be calibrated ■ Short response time ■ Linear output ■ Wide operating temperature range

■ Very accurate ±0.5°C

■ Recommended for measuring a range of temperatures

■ ±1˚C (chilled products) and ±2˚c (frozen products) accuracy

■ Very wide temperature range

■ Very high response time

■ ±0.5°C accuracy ■ Easily calibrated

Advantages

Temperature

0.02°C to 0.2°C drift per year

0.5°C to 1°C drift per year

<0.1% drift in 5 years for platinum

model <0.2°C per year 0.5°C to 1°C drift per

year

Stability

Table 1: Types of thermometer

*Required time in seconds it takes to reach 63.2% of the temperature difference from initial to final product temperature reading

10

Figure 1: The most common sensors used in thermometers in the food industry

a) Thermistor

c) Resistive temperature detector

b) Thermocouple

d) Infrared thermoelectric

e) Bimetallic device


11

4 . THERMOMETER SELECTION

Temperature measurement of food products can be taken by a probe thermometer or infrared thermometer.

The choice of thermometer depends on whether the measurement is to be taken by the non- destructive or destructive method.

The thermometer selected should: ■ reach 90% of final reading in less than three minutes

■ have less than ±0.5°C accuracy at –20°C to 30°C temperature range

■ change by no more than ±0.3°C when operated at –20°C to 30°C temperature range

■ have at least one digit after the decimal point on the display reading

■ be robust, shockproof and waterproof

■ be easy to clean and allow good thermal contact with the tested food

■ be operated by a dry cell battery and have a means of warning when the battery needs replacing

■ suit the target measurement (for example, robust rigid stem with a sharpened point for insertion into product and flat head for between–pack measurements).

4.1 NON-DESTRUCTIVE TEMPERATURE MEASUREMENTNon-destructive testing using either a probe thermometer or an infrared sensor is rapid and can be done without unduly disturbing the food product. However, because the temperature being measured is only of the outside of the package or carton, there could be up to 2°C difference between that and the true product temperature.

Probe thermometer

To obtain the best result place the probe thermometer between boxes on a pallet or between packages inside a carton. Use sufficient pressure to ensure good thermal contact, and sufficient length of inserted probe to minimise conductivity errors. Use a probe with a flat surface for good surface thermal contact, low thermal mass, and high thermal conductivity (Figure 2). The probe must be waterproof (IP 65).

Figure 2:Handheld NTC probe thermometer measuring the temperature between two frozen pizza boxes


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Infrared thermometer

Infrared thermometers come in a range of models, and selection of the right model could mean the difference between accurate and false temperature measurement, particularly of chilled and frozen packaged foods.

It must be remembered that the visible laser beam emitted by infrared thermometers is purely an aid for aiming – the laser beam is not measuring anything. The visible beam indicates the centre of a measuring spot which emits the infrared energy that is measured by the thermometer to arrive at a temperature reading. The size and coverage of that measuring spot is critical to accurate temperature measurement.

An understanding of the ability of the measured product to emit infrared energy is useful. This energy is measured as emissivity, which is expressed as a value – from 0 for a mirror, to 1.0 for a black surface. The emissivity value of most unpackaged foods is in the 0.9 to 0.95 range (Table 2), but if the food packaging has a reflective surface an accurate temperature cannot be taken unless steps are taken to artificially darken the surface of the packaging. Dark coloured objects give the most accurate temperature reading but reflective objects may reflect infrared light back to the thermometer, which will skew the temperature readings.

The majority of infrared thermometers are preset in manufacture at 0.95 emissivity which should cover all chilled or frozen foods. However, if the packaging has a reflective surface, or is made of bright metal, a portion of the object should be covered with black tape. The black tape should be allowed to come to the ambient temperature of the object before a reading is taken. Use the black tape as the target for the temperature reading.

For liquid objects, stir the liquid and then immediately take the temperature reading. For the most accurate readings the thermometer should be the same temperature of the ambient or surrounding temperature.

As an alternative to the above process for darkening a reflective surface, there are infrared thermometers available that allow the user to adjust the emissivity value of the target surface. If this option is taken, the following emissivity value chart will be useful.

Material Emissivity value

Food – unpackaged 0.9–0.95

Air 0.9

Aluminium – oxidised 0.2–0.4

Aluminium – blank 0.04

Aluminium – paint 0.45

Ice 0.98

Glass pane 0.85

Rubber 0.95

Paper – white 0.68

Water 0.93

Plastic – acrylic, clear 0.94

Plastic – white and plastic coated paper 0.84

Paper and cardboard box 0.81

Wood 0.9

Table 2:Emissivity values of objects likely to be found in food packaging


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These are the main considerations before investing in infrared technology:1. Is the thermometer going to be used in a single-product environment – for example, a

loading dock in a distribution centre that handles just one product and nothing else?

The infrared thermometer for this task is one which has the emissivity value of the product preset by the thermometer manufacturer. There is a variety of preset infrared thermometers available. The value cannot be altered and cannot be used effectively for any other product that may have a different surface colour or texture.

2. Is the thermometer going to be used to measure the temperature of a variety of products in a variety of packaging – for example, in a typical cross-dock?

The infrared thermometer for this task is a model in which emissivity values can be selected by the operator to match individual products. This is an alternative to the black tape procedure covered above.

3. What distance will the infrared thermometer be from the product being measured?

This is called the distance-to-spot ratio, and infrared thermometers are sold with this ratio preset during manufacture.

Having determined which of these thermometers will best suit the job, the next choice is either a laser thermometer that emits a single laser point, or one that emits dual laser points.

The distance-to-spot ratio

An accurate temperature reading depends on the distance the infrared thermometer needs to be from the object being measured. This is called the distance-to-spot ratio (Figure 3). The bigger the ratio, the farther away you can stand from the target object while still getting a good reading. For example, with a 30:1 ratio you can be standing 30 inches away from your target and still accurately measure an area of one square inch, or you can be standing 30mm away from your target and still accurately measure an area of one square millimetre.

For an accurate temperature reading, the object being measured should fill the field of view of the infrared thermometer. For example, to accurately measure the temperature of a turkey leg the infrared measuring spot must be totally filled by the leg.

Single point infrared thermometer

The single point infrared thermometer emits a laser beam that shows the centre of a measuring spot with various degrees of allowance for parallax errors.

The measuring spot must be full size on the object being measured, and clear of interference from competing surfaces. In small objects particularly it might be difficult to have a full size measuring spot, and if that happens, the temperature will be represented as a mean value of the surfaces emitting infrared energy, thereby resulting in distorted measurements.

Dual point infrared thermometer

A twin point infrared thermometer emits two laser beams that provide a visual check of the measuring spot. The infrared capture of energy occurs between the two laser beam points on the product. This is more accurate than a single point infrared thermometer.

It is recommended that infrared measurements be taken close to the stipulated distance-to- spot ratio to reduce the effect of ambient light and must not be greater than the target surface area. Some dual point infrared thermometers are designed to cross their beams at a certain distance (stipulated in the device manual). Commonly, the point where their beams cross is the same as the distance-to-spot ratio nominated by the device.

A thermometer with a 30:1 ratio measuring spot means that with the thermometer held at any distance from the object, the spot ratio of 30:1 will allow the user to determine the size of the infrared measuring spot. For example, if the thermometer was 30 metres away from the object, the measuring spot would be one square metre in size.


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Figure 3: Distance-to-spot ratio in infrared thermometers

a) Single point infrared thermometer

b) Dual point infrared thermometer

c) Dual point infrared thermometer with crossover laser beams

2 31

122436

Laser beam

Distance (mm, m, inches, feet)

Measuring spot(mm, m, inches, feet)

Laser beam

2 31

12

24

36

Laser beam


Measuring spot(mm, m, inches, feet)


10.5Measuring spot(mm, m, inches, feet)

6

12

24

2


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Laser thermometer with crosshair sight

Infrared thermometers equipped with precise crosshair targeting confirm the measuring spot with an accurate crosshair pattern, whose dimensions precisely match those of the measuring spot. However, these thermometers are mostly employed in medical and similar scientific applications, and not usually in the food industry.

Thermometer operation

The following steps should be taken to ensure the infrared thermometer gives the most accurate reading:

1. Find the thermometer’s distance-to-spot ratio which is listed on the thermometer and hold it close enough to the target so the thermometer reads only the area that is to be measured (Figure 3).

2. Compensate for reflective objects and low emissivity (Figure 4).

3. Steam or dust can affect the accuracy of infrared thermometers.

4. Keep the lens of the thermometer clean and free of scratches.

5. Allow the sensor time to reach the surrounding temperature (could be up to 20 minutes), especially if there is a big difference between where the thermometer is stored, and the target temperature location.

Figure 5 and Figure 6 illustrate the correct methods of taking infrared temperature measurements.

Figure 4:Comparison between infrared measurement of low and high emissivity surfaces and how to compensate for reflective surfaces with electric tape

Placing black tape on the measurement spot is the best way to obtain the correct surface temperature on reflective or silver surfaces. Allow sufficient time for the tape to reach the same temperature of the product before using an infrared thermometer.

Metal reflective surface Reflective surface

A dark surface absorbs radiation heat from the surrounding medium/air

A reflective surface rejects or reflects most of the radiation heat

Reflected Reflected

Incidentradiantenergy

Incidentradiantenergy

Absorbed Absorbed


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Figure 5:Example: Measuring the temperature of yoghurt in a small pot using an infrared thermometer

Figure 6:Example: Measuring the temperature of sausages in a foam tray using an infrared thermometer

a) Top surface is considered as a reflective surface and there is also a gap between it and the product (do not measure on locations where there may be space between packaging and the product).

b) Measure on the bottom (or wall) surface.

a) Do not aim the laser at the tray surface material. The diameter of the entire measurement spot must always be on the item to be measured.

b) Aim the laser at the top surface of the sausages through the packaging film.


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4.2 DESTRUCTIVE TEMPERATURE MEASUREMENTWith the destructive method, a pointed probe thermometer is inserted into the product or pressed firmly into its side. This method is considered the best and most accurate for measuring the temperature of non-bagged products.

Probe thermometers are not designed to penetrate quick frozen foods. Therefore, it is necessary to make a hole in the product in which to insert the probe. The hole is made by using a pre-cooled sharp pointed metallic device such as an ice punch, hand drill or an auger. The diameter of the hole should be the same as that of the probe, to ensure a close fit.

The depth of the probe will depend on the type of product. Where product dimensions allow, insert the probe to a minimum depth of 2.5cm from the surface of the product. For smaller products, the probe should be inserted to a minimum depth from the surface of three or four times the diameter of the probe.

Where it is not possible to make a hole in certain foods, such as diced vegetables, the internal temperature of the food package should be determined by insertion of a suitable sharp- stemmed probe.

In general, the probe thermometer should be accurate at the preferred temperature range, instantly readable and fitted with a thin probe that slides easily into the product.

Temperatures that may be in dispute can only be proven by a destructive temperature check, using a probe thermometer for which calibration, accuracy and limitation tolerances can be confirmed. This action would be necessary to avoid load rejection and potential liability claims.


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5. MEASUREMENT PROCEDURES AND TECHNIQUES

Measuring and monitoring the air temperature can indicate whether refrigerated equipment is functioning correctly, but it cannot reflect food product temperatures as these depend on many parameters such as thermal properties and packaging of the food and flow of air around the product.

The accuracy of temperature measurement depends on how and where the probe thermometer is placed. If wrongly placed, the reading will be inaccurate.

The only accurate food temperature is core/pulp temperature because the surface temperature may be warmer or cooler than the temperature in the rest of the food.

The following steps will ensure the most accurate temperature measurement:1. Ensure the device has been properly calibrated (section 5.1).

2. Verify the display.

3. Check the battery and if replacing, always calibrate.

4. Stabilise the temperature of the sensor (the time required to get a stable temperature reading depends on the thermometer response time – see Table 1) and pay attention to the temperature delay and radiation heat.

5. Pre-cool the thermometer prior to the test to equalise the temperature of the probe to that of the product’s surrounding air temperature. This avoids heat being conducted from the probe to the product which can result in inaccurate temperature measurement.

6. Clean the thermometer before and after the test. Wash it with cool soapy water to remove food particles and grease, rinse it with clean hot water or clean with alcohol wipes. Allow to air dry or wipe it dry with a clean cloth or paper towel.

EU Standard DIN EN 13486

5.1 CALIBRATIONThermometers should be calibrated and certified at six-monthly, or 12-monthly intervals, or:

■ when any measurement problem is detected

■ when the battery is changed

■ whenever conditions relating to the cold chain process might require more frequent calibration

■ in accordance with manufacturer recommendations.

Accuracy must be checked in iced water/melting ice or in a commercial grade temperature bath.

A calibration check can be done regularly internally, using the iced water procedure, or the calibration can be carried out by an external provider. However, thermometer repairs or adjustments are best carried out by an external provider with expertise in servicing thermometers.

Infrared sensors should be calibrated every two years, or in accordance with manufacturer recommendations. A device known as a comparator cup, that checks an infrared hand-held thermometer against a known reference thermometer can also be used.

The maximum calibration uncertainty for temperature measurement devices should be ±0.1°C to ±0.5°C depending on the class. Table 3 shows the calibration uncertainty for the verification measurement range with a one-year calibration interval.

SS-EN 13486 European standards 2001/2002, EN 60751: 2008


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The most accurate thermometer calibration is performed on dedicated calibration instruments fitted with a calibration bath. Calibration instruments offer stability, uniformity and flexibility.

These are located at testing facilities. Some thermometer manufacturers offer calibration services.

A simple and quick manual calibration can be performed using iced water and boiling water to find the offset. Although convenient, this is not as accurate as dedicated calibration instruments.

It is important to use both the ice point and boiling point methods to ensure the thermometer is accurate in its upper and lower ranges.

Ice point calibration1. Fill a container with crushed ice.

2. Mix enough chilled water to produce a slurry, not enough to float the ice (Figure 7).

3. Stir the slurry vigorously.

4. Insert the thermometer probe into the iced slurry for at least three minutes.

5. Record the reading.

6. For accuracy, take a second reading at least one minute later. The results should be within 0.5°C of each other. If not, replace or service the thermometer.

7. Record the readings.

Type ThermocouplesResistance thermometers

(metallic sensors, resistive temperaturedetectors, thermistors)

Class 0.5 1.0 2.0 AA A B C

Accuracy/ tolerance °C ±0.1 ±0.2 ±0.5 ±(0.1+0.0017

|T|*)±(0.15+0.002

|T|*)±(0.3+0.005

|T|*)±(1.2+0.005

|T|*)

*|T| is the numerical value of the temperature in °C irrespective of the sign.

Figure 7:Ice point calibration (A full slurry of ice in water is required.)

Boiling point calibration1. Heat a saucepan of water to a continuous rolling boil.

2. Insert the thermometer probe into the boiling water for three minutes.

3. Record the reading.

4. For accuracy take two readings, two minutes apart. The results should be within 0.5°C of each other. If not, replace or service the thermometer.

Table 3:Criteria for selection of working-standard thermometers


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Mechanical calibration

Some thermometers come with a mechanical calibration unit.

If there is no access to a calibration unit of any kind, return the thermometer to the manufacturer for calibration.

5.2 WHERE TO PLACE THERMOMETER PROBE IN FOOD

a) Chilled and frozen products

The most accurate method of measuring food temperature is the destructive method, where the thermometer probe is inserted inside the product.

Extra care is needed for meat products, because bone, fat and gristle have different thermal properties and heat transfer rates (Table 4).

Meat and poultry are highlighted because these foods are the most refrigeration intensive and are also subject to the most stringent standards and regulatory scrutiny.

If the food is irregularly shaped, the temperature should be checked in several places. Ensure that the probe does not penetrate the packaging because this could contaminate the contents and damage the sensor to the point where it delivers incorrect readings.

The probe should be disinfected before and after measuring products. Use dedicated disinfecting cloths or hold the sensor in boiling water and wipe with clean, disposable paper.

At critical control points, such as a loading dock that is exposed to ambient airflow, the temperature of the outer surface of the package cannot be relied upon. Correct temperatures can only be taken in a refrigerated space. At loading docks, temperature measurement can be taken with a probe placed between two packs while they are still in the truck or trailer. Leave the probe in place for at least one minute before reading the results..

Figure 8:Measuring temperature of fresh chilled products

a) Standard pulp thermometer (bimetallic) probe

b) Measuring the pulp temperature of an apple using a digital thermometer with pointed probe

c) Non-destructive measurement by placing the probe between two packaging units


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Figure 9:Some right and wrong thermometer placements

a) With the sensor measuring the end of the leg (bone)

c) Measuring the surface of the meat beneath the wrapping

b) Measuring the surface temperature of the wrapping film and not touching the leg meat

Product Probe placement

Beef, pork, lamb Centre of the thickest part away from bones, fat and gristle

Hamburger, steak or chops Thickest part away from bone, fat and gristle

Whole poultry Thickest part of the thigh away from bone

Poultry pieces Thickest part away from bones or insert sideways

Ground meat and poultry Thickest area (try to get the centre temperature and for thin products, insert the probe from the side until it reaches the centre)

Fish Thickest part

Vacuum-packed products Between two packs

Deep-frozen products Between the deep-frozen products

Cut meat products, cheese Between two packs

Table 4: Probe insertion into whole protein food


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b) Fresh produce

Measuring the temperature of horticultural products can be complicated because of variations in product size, shape and type, as well as the type of packaging. The measurement method allowable or selected by cold chain practitioners also has to be taken into account.

There are two temperature measurement methods for fresh produce – pulp (core) and surface.

Pulp measurement process ■ Pulp measurement is a destructive method in which a pointed probe thermometer is

inserted into the product. This is considered the best and most accurate method for measuring the temperature of non-bagged products. However, this method is not always appropriate or allowable in some cold chain processes because it means that at least one piece of produce is damaged for each probe measurement.

■ As an alternative, the interface method, using a flat probe inserted between two packed products or two individual pieces of fruit or vegetable can also be used. In this method, pressure has to be applied to ensure that the probe is completely covered by the surface of the product – no part of the probe can be exposed to air. This means product destruction may occur due to marks or bruising left by the probe.

Surface measurement process ■ Surface measurement is usually a non-destructive method using either an infrared

thermometer or a flat probe thermometer. When using an infrared thermometer, steps outlined in section 4.1 should be followed. When using a flat probe thermometer, the interface method is used, requiring the application of light pressure, with the probe kept in position for a few minutes until the temperature is stabilised.

■ The pulp temperature of the product can then be determined from the surface temperature provided that temperature calculation formulas exist for each type of product. If single use temperature indicator technology is used, temperature abuse can be detected during the journey and no produce is damaged. With this technology, measures should be taken in different zones to ensure delivery at the right temperature. If the average reading is above the maximum allowed temperature a pulp temperature check becomes essential.

■ For produce on a pallet, trailer air fluctuation can impact on the produce at the top of the pallet so these should be avoided in any temperature measurement.

■ Ultimately, for temperature measurement of whole fruit or bagged produce, the cold chain practitioner must determine which method to use – core or surface temperature checking – subject to the processes in place for such shipments.


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The pulp temperatures and acceptable receiving temperatures for fresh fruit and vegetables are listed in Cold Food Code – Book 4 (Guiding principles of fresh produce transport and storage).

5.3 FACTORS THAT AFFECT MEASUREMENT ACCURACYSome factors cause sensor measurement errors.

■ Stem losses and thermal shunting can happen in thermocouple and resistive temperature detector sensors due to their large mass and could affect the temperature of the tested food. With thermocouples or thermistors, if the mass is minimised by reduction of the wire diameter, the wire will be more susceptible to annealing, strain and shunt impedance. Wire diameters are determined during the manufacture of the thermometer.

■ Radiation heat is the emitted energy from solar radiation in the surrounding medium which is transferred to the product surface and mainly happens when the surface temperature is measured using infrared sensors.

■ Frictional heating can occur when two bodies slide against each other causing heat generation due to friction.

■ Heat transfer can occur in the electronic component of surface-mounted technology sensors and result in an inaccurate temperature measurement.

Measurement method Time Accuracy Pros Cons Note

Pulp temperature

Less than 10 seconds

Very accurate depending on probe sensor

accuracy

Fast and reliable

Produce is damaged and

wasted

Thermometer with long probe and sharp tip is

required

Interface between two products with

pressure

Up to three minutes for positioning

and then stabilising the temperature

An average of core and

surface temperature

Acceptable accuracy

Produce can be damaged by the probe

being jammed between products

The probe should be covered by

product surfaces, with no exposure

to air

Interface between two products with slight pressure

One minute for temperature stabilisation

Acceptable accuracy if

carried out in refrigerated

area

Products are not damaged

or wasted

Not accurate if part of

the probe is exposed to air

A flat end probe is required to avoid

damaging the product or the

pack

Surface temperature Immediate

Not accurate especially

outside the refrigerated

area and taken from a long

distance

Quick and easy – products are not touched

Not accurate and steps in section

4.1 must be followed if

using infrared thermometer

Measurements should be taken at different

locations in a pallet or trailer

Table 5: Fresh produce comparisons depending on measurement method


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6 . IMPORTANCE OF TEMPERATURE CONTROL

Food temperature should be monitored and controlled in the cold chain to meet expected standards of food safety.

Any change in the recommended product temperature affects the quality and chemical properties (water content, acid and pH) of the food and its shelf life. Rising temperature increases the growth rate of microorganisms and pathogenic food poisoning bacteria, reducing the food shelf life.

Foods at risk (raw and cooked meat, dairy products, seafood, processed fruits and vegetables, cooked rice and pasta, processed food containing eggs, nuts, beans or other protein) need to be kept in their recommended temperature range. Table 6 shows the standards for the shelf life of meat products at different temperatures. Table 7 is a guide for 2 to 4 hours of temperature abuse (above 5°C) of foods at risk. If the product is kept at 5°C or higher for more than 2 hours it should be discarded..


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Total time above 5°C Required action

Less than 2 hours Refrigerate or use immediately

Between 2 hours and 4 hours Use immediately

More than 4 hours Discard

Class Type Temperature Beef Lamb Pork Chicken

Un

pro

cess

ed

Carcass <7°C 7–10 days 6 days 5 days -

Carcass/quarters

0–2°C 3–4 weeks 10–13 days - -

Carcass <5°C - - - 2–3 days

Parts <5°C - - - 1–2 days

Primal cuts unpackaged

<5°C 3–5 days 3–5 days 3–5 days 1–2 days

Retail cuts overwrapped

<5°C 4–5 days 3–4 days 3–5 days 3 days

Retail ready mince unpackaged

<5°C 1 day 1 day 1 day 1 day

Retail ready mince overwrapped

<5°C 4 days 4 days 4 days 4 days

Mince frozen–18°C to

–12°C 2–3 months 2–3 months 2–3 months 2–3 months

Portions frozen

–12°C 8 months 12 months 6 months 9 months

–18°C 18 months 18 months 10 months 18 months

Pro

cess

ed(v

acu

um

pac

ked

)

Primal cuts 0°C 20 weeks 12 weeks - -

<5°C 5 weeks 4 weeks 3 weeks 10 days

Portions <5°C 3 weeks 3 weeks 2 weeks -

Machine diced and sliced

<5°C 3 weeks 3 weeks 3 weeks 7–9 days

Hand diced and sliced

<5°C 2 weeks 2 weeks 2 weeks 5–7 days

Mince raw <5°C 7 days 7 days 7 days 7 days

Table 6:Summary of shelf-life guidelines for packaged meat and meat products

Table 7:Length of time foods at risk can be held safely at temperatures above 5°C


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No

No YesHas the sensor been calibrated in the past 6 months?

Measurement method

Check the battery, accuracy and speed of the thermometer

Do the calibrationSee section 5.1

Put the thermometer between two product trays/packages

See figure 8d

Find distance-to-spot ratio and field of view to

determine the right distance and vision angle for an accurate measurement

See figures 3, 4, 5, 6

Do the measurement (avoid the reflective

packaging surface if using an infrared sensor)

Check thetemperature limits

Destructive method(be sure the probe is cleaned) Non-destructive

method

Change thetrays/packages

YesAllow time for the

infrared sensor to reach the surrounding temperature

Is there an ice layer on the film?

No YesIs the temperature within the limits?

Reject the load Unload/load processEnd

Insert the probe and measure product core temperature

See section 4.2 and figure 8aThermometer Infrared sensor

Figure 10:Flowchart for temperature measurement process

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Australian

Part 1

COLDFOODCODE

Basic principles of thermometers and how to use them

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